1. Field of the Invention
The present invention relates generally to devices that transmit acoustic or stress wave signals. More particularly, the present invention relates to devices that transmit acoustic or stress signals through an elastic media. Still more particularly, the present invention relates to acoustic transmitters that transmit acoustic or stress signals via well tubing.
2. Description of the Related Art
The recovery of subterranean hydrocarbons, such as oil and gas, usually requires drilling boreholes thousands of feet deep. In addition to an oil rig on the surface, production tubing extends downward through the borehole to hydrocarbon formation. The tubing may have horizontal, or lateral bores which incorporate valves to control the flow of hydrocarbons or other well fluids. To efficiently operate this complex production system, well site personnel often require reliable "real time" data regarding borehole conditions. For example, knowing downhole pressure and temperature is vital in determining whether production is proceeding within permissible operating parameters. With the prevalence of multi-lateral drilling, characterizing the well fluids, using data such as resistivity measurements, plays an important role in deciding which valves to actuate in order to maximize hydrocarbon recovery. Because accurate well bore data is essential to effective management of well site operations, reliable means is required to transmit accurate borehole environment information to the surface.
Several approaches have been employed with limited success to transmit downhole telemetry data to the surface. Linking downhole instrumentation to the surface with wiring has proven exceedingly expensive and unreliable due to the corrosive fluids and high ambient temperatures often found in the well. Electromagnetic radiation has been utilized as a transmission media. However, the non-uniformity in conductivity has prevented wide spread use of this approach. More common is the practice of transmitting data using pressure waves in drilling fluids such as drilling mud, or mud pulse/mud siren telemetry. However, the low baud rate normally produced by mud pulse telemetry transmitters limits the ability of well site personnel to analyze and respond to well conditions. Further, this approach is not available for production tubing because no drilling fluids are present.
Telemetry utilizing acoustic transmitters in the pipe string, such as a mandrel or production tubing, has emerged as a potential method to increase the speed and reliability of data transmission from downhole to the surface. When actuated by a signal such as a voltage potential from a sensor, an acoustic transmitter mechanically mounted on the tubing imparts a stress wave or acoustic pulse onto the tubing string. Because metal pipe propagates stress waves more effectively than drilling fluids, acoustic transmitters used in this configuration have been shown to transmit data in excess of 10 BPS (bits per second). Furthermore, such acoustic transmitters can be used during all aspects of well site development regardless of whether drilling fluids are present.
Despite the promise of acoustic transmitters as an approach to increase data transmission rates, tubing string within the borehole often develops mechanical stresses that can render prior art acoustic transmitters inoperative. Referring to FIG. 1, a prior art transmitter 101 is disposed downhole. Prior art transmitter 101 includes a piezoelectric stack 102 having a plurality of elements and a member 103 having a first end 104 and a second end 105. The first end 104 and second end 105 of the member 103 are solidly connected to production tubing 106. Member 103 has an annular recess 107 that captures the piezoelectric stack 102. A substantially axial interference fit exists between the piezoelectric stack 102 and the recess 107 in order to induce an axial loading that tends to compress the elements of the piezoelectric stack 102 together; i.e., the piezoelectric stack 102 is under a compressive loading as indicated by arrows 108.
As a voltage differential is applied to the piezoelectric stack 102, the elements of the piezoelectric stack 102 expand axially in the recess 107. The piezoelectric stack then contracts when the voltage across it returns to zero. As long as the piezoelectric stack 102 is compressed within the recess 107, piezoelectric stack 102 expansion will axially displace the first end 104 of the member 103 with respect to the second end 105 of the member 103 and thereby induce a controlled stress onto the tubing 106. This controlled stress generates waves in the tubing that are propagated to the surface. As such, by applying voltage differentials to piezoelectric stack 102 in a controlled manner, waveforms are generated that transmit data to the surface.
For the acoustic transmitter to function properly, the recess 107 must maintain the compressive loading 108 of the piezoelectric stack 4 within a limited range. If stresses in the tubing 106 push the first end 104 and second end 105 of the member 103 toward one another, the resulting compressive loading may be too severe and result in a "locking up" of the piezoelectric stack 102 by preventing the piezoelectric stack 102 from expanding as voltage is applied. On the other hand, if stresses in the tubing 106 pull ends 104, 105 apart, tensile loading 109 results. This tensile loading 109 reduces the compressive loading 108 of the piezoelectric stack 102. When tensile loading 109 is sufficiently high, the elements of the piezoelectric stack 102 separate and are no longer able to generate stress signals on tubing 106.
Unfortunately, compressive and tensile loading are often encountered during normal hydrocarbon drilling and production. Referring to FIG. 2A, a tubing 201a having an acoustic transmitter 204a is suspended within a borehole 202a from a rig 203a. Where tubing 201a extends for several thousand feet, a prior art transmitter 204a interposed in that span can be subjected to significant tensile loading, T. Referring now to FIG. 2B, if a packer 205b were released in the middle of such a long expanse of tubing, a prior art transmitter 206b located above the packer 205b may encounter compressive loading, C. Moreover, as shown in FIG. 2C, wells that have deviated tubing 207c present unique problems because it is impossible to predict which sections of tubing 207c will be subjected to compressive loading and which sections of tubing 207c will be subjected to tensile loading. Therefore, even under normal operating conditions, prior art transmitters can suffer from complete signal loss because of piezoelectric stack "lock up" or separation.
Additionally, when prior art acoustic transmitters are operated at well sites, undesirable multiple resonances are often displayed during band sweeps. That is, transmissions over particular frequencies generate amplitude spikes that complicate the monitoring of well bore data. Moreover, the locations of the resonance frequencies vary with the unique configuration of each well site.
These and other problems have prevented the oil and gas industry from utilizing fully acoustic transmitters. As such, there exists a need for an improved acoustic transmitter. The present invention overcomes the deficiencies of the prior art.